Component packaging and assembly

ABSTRACT

A packaging layer ( 200 ) for a wafer level assembly is fabricated from a glass material comprising both inorganic and organic components. This allows matching between the coefficient of thermal expansion of the packaging layer and that of other materials in the wafer assembly, particularly electrical interconnect materials. It is also possible to introduce properties to support such methods as photolithographic and low temperature processing of the packaging layer. This can improve fabrication accuracy and allows the packaging layer to be used with structures in a wafer assembly which might be damaged by high temperature processing, such as active optoelectronic devices and integrated circuits. Another major advantage is that the glass material can be used to provide optical characteristics as well as mechanical protection. The refractive index and other optical properties can be preselected and thus the glass material can be used for instance for waveguiding and index matching.

This application is the US national phase of international applicationPCT/GB2003/004129 filed 19 Sep. 2003 which designated the U.S. andclaims benefit of EP 02256515.4, dated 19 Sep. 2002, the entire contentof which is hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates to component packaging and findsparticular application in wafer-level packaging of, for example,electrical and/or optical components such as lasers and associateddevices.

2. Related Art

Component packaging is used in semiconductor-based technologiesgenerally to protect or support a component or an assembly of componentsfor handling or further processing. Packaging can potentially be done atdifferent levels, from individual items up to finished assemblies. Forinstance, a sub-assembly of components can be packaged together so thata function of the sub-assembly can be tested without waiting for thefinished equipment.

Various types of protection can be offered by packaging, includingmechanical and chemical. For example, a passivation layer can be used toprovide environmental protection for any active and passive componentsintegrated on a substrate. Such a passivation layer can be providedduring an assembly process, as an intermediate layer of a wafer levelassembly, or deposited as a final step in wafer level processing. Inanother example, a planarisation layer can be used to improve a surfacefor subsequent layers or devices. Planarisation layers are used forexample to smooth the surface of a substrate which is otherwise uneven,such as ceramic and composite surfaces such as alumina and LTCCsubstrates or plastics. Unpolished substrate surfaces can be planarisedto achieve good surface quality and surfaces already carrying otherstructures, such as interconnect material, can be smoothed over by aplanarisation layer to present a flat surface in spite of the presenceof the structures.

Wafer level packaging is becoming known as an attractive method ofpackaging low to mid density devices for several reasons: cost, size andease of testing.

Cost is the largest force driving wafer-level packaging. Usingsimultaneous batch integration processing, an entire wafer or substratecan be packaged instead of packaging each device. Wafer level packagingreduces the number of steps involved in packaging, potentiallyeliminates the use of underfill and allows for centralized processingduring fabrication. Further, packaging of the wafer allows for a highdegree of process integration, due to the use of technologies such asthin film and lithography, which decreases cost. Centralized packagingduring fabrication also reduces packaging time and inventory, sincedevices are no longer packaged separately prior to assembly.

Device size is also a driving force for wafer-level packaging. The sizeof a wafer level packaged device can be much the same as packagedsemiconductor chips.

From the testing point of view, wafer-level packaging has a majoradvantage. Testing at the wafer-level (which is intermediate testing ofthe device functionality to decide which particular devices are going tobe finally packaged and used as an end product) can reduce test costs byas much as 50%, requiring both less capital and reducing the number oftest steps.

However, known wafer-level packaging is not suitable for use in thefabrication, assembly and packaging of all components and devices. Forexample, it is not used for active optical devices and components suchas tunable and/or external cavity lasers.

Conventional approaches for wafer level integration and the fabricationof “build-up layers” on an integration substrate are based on thedeposition and patterning of organic materials such as Dow Chemical'sSilK, or the chemical vapour deposition (CVD) of metal oxide typecoatings.

A known problem of purely organic materials is that their co-efficientof thermal expansion (CTE) is high compared to that of materials such asmetals and semiconductors used in substrate-based integration forexample for interconnects and other aspects of optoelectronicoperations. A typical CTE of organic materials is 60 ppm (parts permillion per ° C.) or higher. A significant mismatch in CTE between alayer and material it is in contact with tends to cause stresses at theinterface. In addition, organic materials lack thermal stability, whichmay limit a device's long-term stability and exclude some manufacturingtechniques such as soldering and metallization.

On the other hand, CVD is a high temperature process, which limits theselection of substrate materials and the type of electronic oroptoelectronic devices which can be assembled to the substrate beforeCVD inorganic film deposition. Furthermore, the processing of openings(assembly holes) is time consuming and deep structures are relativelycomplicated to fabricate.

BRIEF SUMMARY

According to a first aspect of the present invention, there is provideda substrate-based assembly for carrying optical and/or electricalcomponents, the substrate-based assembly comprising at least onepackaging layer, wherein the packaging layer(s) comprises a glassmaterial having both organic and inorganic components.

The “substrate-based assembly” may comprise a wafer assembly in which atleast one device or component is to be supported on a wafer of materialas substrate. A “substrate-based assembly” might already have a deviceor component mounted thereon but the phrase encompasses a substrate pluspackaging layer prior to, or in the absence of, mounting of any deviceor component.

A “glass material” in this context is used in the usual way to mean anamorphous or non-crystalline solid. References elsewhere in thisdescription to a “hybrid glass material” and the like are intended torefer to a glass material having both inorganic and organic components.

In an embodiment of the present invention, because of itsorganic/inorganic nature, it is possible to select a glass material forthe packaging layer which has one or more particular properties orfunctions. These may be mechanical. For example, the adjustment of theorganic/inorganic ratio makes it possible to tune the values for one ormore of: CTE; hardness; stress modulus (known as substrate bow); andthermal stability.

For example, if the concentration of inorganic materials is increased:

-   -   the CTE decreases    -   the hardness increases    -   the stress modulus increases    -   thermal stability typically increases.

If the concentration of organic materials is increased:

-   -   the material softens and becomes more elastic    -   the CTE increases    -   the thermal stability decreases.

The CTE of suitable organic materials is typically 50 ppm or more andthe CTE of inorganic materials (e.g. glass, silica, alumina etc.) istypically just a few ppm or less. Hence it is available to tune the CTEover a significant range by adjusting the inorganic/organic ratio in thematerial. For example, the CTE is 100 ppm or more in materials having anepoxy concentration of about 70% or more while alumina has a CTE of 6.7ppm and silica has a CTE of 0.5 ppm.

A substrate-based assembly according to the first aspect of the presentinvention may further comprise electrical interconnect material.

It will usually be preferable that the value of the CTE for the hybridglass material used as a packaging layer approaches that of theelectrical interconnect material. For example, suitable interconnectmaterials that might be used are copper and aluminium, in which case thecloser the CTE of the hybrid glass material is to the values for copperand aluminium the better. In general, it is preferable that the CTE ofthe hybrid glass material should not differ more than 15 ppm from theCTE of the interconnect material.

It may also or instead be preferred that the value of the CTE for thehybrid glass material used as a packaging layer approaches that of thesubstrate material. For example, it might be preferred that the CTE ofthe hybrid glass material should not differ more than 15 or 20 ppm fromthe CTE of the substrate material.

The electrical interconnect material might be present for example asstructures such as contact pads for bump bonding or for wire bonding. Asub-assembly comprising an embodiment of the present invention may beused as an integration level for bump bonding in which a bump ofconductive material is positioned on a pad of interconnect material, orindeed double bump bonding where a double layer of solders (bumps) iscreated on top of interconnect material that has been provided in thesub-assembly.

Concentration levels of inorganic and organic material components can bevaried considerably in the glass material, providing a potential rangeof CTE values for example from 3 to 100 ppm. In the glass material, aninorganic matrix can be provided at least in part by any metal alkoxideor salt that can be hydrolysed, all of these being appropriate inorganicnetwork formers, including those based on groups 3A, 3B, 4B and 5B ofthe Periodic Table, such as silicon dioxide, aluminium oxide, titaniumdioxide and zirconium oxide.

Functional organic moieties can then be used to modify the inorganicmatrix. In general, the glass material of the substrate-based assemblywill preferably include an organic component which polymerises bycross-linking. It might for instance be an organic component whichpolymerises under thermal or photo treatment, such as the functionalhydrocarbon compounds comprising acrylates, epoxides, alkyls, alkenes,or aromatic groups which support photopolymerisation.

The hybrid glass material of the packaging layer in a substrate-basedassembly according to the first aspect of the present invention mayadvantageously have lithographic properties. The combination of organicand inorganic properties allows direct photolithographic patterning ofthe glass material. Thus it can be possible to select materials inembodiments of the present invention such that all fabricated structurescan be produced by lithographic processing. This enables accuratepositioning (sub micrometer accuracy) of components such as opticalsub-assembly elements. Electrical interconnects can be created eitherbefore or after deposition of the hybrid glass material and thecomponents can be mounted after the patterning of the hybrid glassmaterial.

For example, the packaging layer of a substrate-based assembly accordingto the first aspect of the present invention might be provided withrecesses during patterning, either as depressions or holes, in relationto which one or more components can subsequently be mounted.

All the inorganic and organic material components mentioned above arevery favourable in terms of their properties and their tunability forwafer-scale integration.

By selection of appropriate components, it is possible to use lowtemperature processing of a hybrid glass material, for instance at lessthan 450° C. or even less than 200° C. or even 150° C. This makes itpossible to integrate the glass onto existing electronic components andcircuitry. In particular, if high processing temperatures are requiredthe organic component content should be kept to a minimum. Lowprocessing temperatures are made possible by using thermal- orphotoinitiators resulting in polymerization of the organic matrix. Thepolymerisation may be achieved through organic carbon-carbon double bondopenings and crosslinking. Known thermal initiators include benzophenoneand various peroxides, such as benzoylperoxide and layroyl peroxide.Known photoinitiators include phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (Irgacure 819) and1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 814). (Irgacure initiatorsare products of Ciba Specialty Chemicals Inc. and “Irgacure” is aregistered trade mark.)

In a novel arrangement, one or more components may be mounted in thesubstrate-based assembly for electrical interconnection using the knowntechnique of “solder bump bonding”. Standard solders such as lead andtin alloys can be used. In this known technique, solder bumps areapplied to a connection surface of the component, or to a supportsurface, the component is placed in contact via the bumps with thesupport surface and then heat is applied. The solder flows to provide arelatively intimate bond and good electrical performance in use. Solderbump bonding has been found to have a very significant advantage howeverin that the components can be accurately aligned by manipulation whilethe bonding material is soft. This can be done during a mountingoperation or subsequently by applying heat to the bonding material.

As well as components or devices, the substrate may also or insteadsupport control electronics such as laser drivers, thermo-elements,signal controllers, buried integrated circuits such as centralprocessing units (CPUs) for embedded software, and their variousinterconnects. One substrate can carry one to several tens ofsub-assembly structures.

Suitable substrates for use in embodiments of the present invention aresilicon, glass, composite materials, ceramics including multi-layerceramics such as alumina and low temperature-co-fired ceramics (LTTC),and even conventional printed circuit board.

The packaging layer might be supported directly or indirectly by thesubstrate. One potential use for a packaging layer comprising a hybridglass material is as a passivation layer and this might be appliedduring, or as a final layer in, wafer processing.

Optoelectronic equipment comprising a substrate-based assembly accordingto an embodiment of the present invention is also encompassed as anembodiment of the present invention. For example, this might be anoptical source such as a wavelength tunable optical source.

In the past, packaging layers have provided only a generally protectiveand/or supporting function in a wafer. They have not contributed tooptical qualities such as confinement of radiation travelling in thewafer. A packaging layer in a substrate-based assembly according to anembodiment of the present invention may have optical characteristics,for instance allowing it to be used to transmit optical radiation in useof the assembly, thus allowing fuller integration of optical devices atwafer level.

In such arrangements, the optical properties of the material of thepackaging layer will be important. In addition to the possibility oftuning the mechanical characteristics described above, it is alsopossible to tune the optical characteristics. Thus a packaging layeritself can be used to have an active optical function in addition to itsassembly role. Optical properties such as refractive index,birefringence, dispersion and dn/dT-value (rate of change in refractiveindex against temperature) can be tuned by selecting a glass materialwith certain composition.

For example, components which can be used to select the above opticalcharacteristics are discussed below:

The relationship dn/dT in the hybrid glass materials usually has a valueranging from −10×10⁻⁵ (1/° C.) to 40×10⁻⁵ (1/° C.). This range can bepotentially made wider by choosing the right material composition.Especially in some applications material dn/dT value of 0 (zero) orpositive dn/dT values would be very attractive to use. Material dn/dTvalue can be adjusted based on the crosslinking density oforganic-inorganic components in the material matrix.

Increasing crosslinking density in an organo-siloxane matrix decreasesthe intrinsic dn/dT value of the material. The materials crosslinkingdensity in an organic-inorganic matrix can be raised using two differentapproaches. A first approach is to increase the molar concentration oftetra functional siloxanes in the matrix, for example by increasing thecompositional concentration of precursors that are capable of formingfour silicon-oxygen bridges to other silicon atoms. A second approach isto increase the amount of multi-functional organic moieties in thematrix and especially components that are able to undergo organiccrosslinking through organic double bond polymerisation.

The intrinsic dn/dT value will be decreased in the siloxane polymer by:

-   -   increasing the concentration of tetrafunctional siloxanes    -   increasing the concentration of thermally or radiation        crosslinkable organic moieties    -   increasing the concentration of non-organic-moieties containing        metal or metalloid components

The intrinsic dn/dT value will be increased in the siloxane polymer by:

-   -   increasing the concentration of bifunctional and monofunctional        siloxanes    -   increasing the concentration of non-crosslinkable organic        moieties

By selection of appropriate inorganic and organic components, it ispossible to tune the refractive index of a hybrid glass material to havea desired value. When using silicon dioxide based inorganic components atypical achievable range for indices is 1.41-1.52 at 1550 nm wavelengthrange. However, if e.g. zirconium, titanium or germanium basedcomponents are introduced to the material higher (close to 1.7 at 1550nm) refractive indices can be achieved. The low refractive indexes canbe achieved by using fluorinated organic moieties in the hybrid glassmaterial.

Although the hybrid glass materials are considered as amorphous andnon-crystalline solids the material may exhibit birefringence with somecompositions. Birefringence is relatively low compared to crystalline orsemi-crystalline glasses and polymers. Typical birefringence values withsiloxane polymer materials may be in the range of 1×10⁻⁵-8×10⁻⁴ and haveto be controlled chemically by selection of the right materialcomponents.

The materials absorption/transmission characteristics can be tuned sothat a specific optical wavelength experiences minimal optical losseswhen propagating in the material. This characteristic (optical loss) isgreatly affected by the material components used and by the synthesisand processing conditions. The hybrid glass materials are synthesized byusing metal alkoxides or salts (such as tetra alkoxy silane or tetrachloro silane), organo functionalized metal alkoxides or salts and pureorganic moieties as precursors. The alkoxides are hydrolyzed andsubsequently condensed by using proper reaction conditions and catalyststo results in metal oxide matrix formation (e.g. Si—O—Si, Ti—O—Ti,Al—O—Al, Ge—O—Ge, Zr—O—Zr). It should be noted that some alkoxide groupscan remain unreacted and adversely affect optical properties of thehybrid glass produced. It is preferable to avoid this situation anddrive the condensation reaction fully to the end. The organic moietiescan also be reacted during the material synthesis with other precursormolecules or they can stay in the material as monomeric or oligomericspecies not bonded covalently to the inorganic matrix components (metaloxide matrix). The choice of the organo modified metal alkoxides andsalts and the purely organic precursors will also greatly affect theoptical properties of the final hybrid glass material. In some cases itmight be preferable to use fluorinated organic moieties to achieve therequired low level in optical losses. The lithographic processing of thesynthesised hybrid glass material will also affect the opticalproperties of the material film or structure. For example, the bakingand UV-exposure conditions will affect final optical properties of thematerials.

The term “organo functionalised” is used above. This is intended to meanan organic (for example a methyl group or benzene ring) is attached to ametal atom (for example silicon). There may be an alkoxide therealready, such as a methoxy group. Organo functionalized however is meantto mean that the organo function does not react away during thehydrolysis and condensation as the methoxy group does.

However, when fabricating for example waveguide structures, thedominating factors for optical losses in the waveguide structure willbe: coupling loss, waveguide shape and surface roughness. The opticalloss for the bulk material itself can be tuned to be below 0.5 dB/cm or0.1 dB/cm or even 0.01 dB/cm depending of course on the specificwavelength(s) used.

Although the use of hybrid glass material gives significant freedom fortuning and optimization of the material's mechanical and opticalproperties, one has to be able to control all of these propertiessimultaneously. All the above parameters have to be taken into accountin designing the material synthesis in order to achieve repeatable,consistent optical materials that lend themselves to further fabricationprocess optimization.

It may be preferred that more than one packaging layer is provided, eachpackaging layer comprising a glass material having both organic andinorganic components. Where the assembly is to be used to carry opticalradiation, it may be preferred that two or more packaging layers havediffering optical qualities. For example, if one packaging layer has adifferent refractive index from another packaging layer, this might beused to support confinement of the optical radiation as it travelsthrough the assembly.

A packaging layer in an embodiment of the present invention might beused for planarisation of a substrate. Depending on the substratematerial e.g., silicon, glass, plastics, composite materials, ceramics(alumina and low temperature-co-fired ceramics), printed circuit boardmaterials (polyimide and FR-4), the substrate may have a surfaceroughness that significantly affects the following fabrication steps.Conventional methods such as polishing can be used to achieve goodsurface quality. However not all the substrate materials are suited forpolishing and furthermore it is a time-consuming process step. In thecase of composite materials, ceramics and plastics it might be necessaryto smooth the substrate surface before optical/assembly structuredeposition. A planarisation layer can be deposited on the substrate tosmooth its surface. Planarisation better than 90%, 95% or even 99% (thatis, thickness uniformity is better than for example 99%) can beachieved, resulting in a substantially uniform surface for subsequentprocessing steps.

A planarisation layer can also be used where a substrate has alreadybeen modified in some way, for instance by the addition of a structure.For instance, the substrate surface may carry electrical interconnectmaterial structures or other structures. This modified surface can becoated with a packaging layer comprising a hybrid glass material. Again,planarisation better than 90%, 95% or even 99% can be achieved,resulting in a uniform surface for subsequent lithography or othersteps.

Just as with other packaging layers, one or more planarisation layersmay have optical characteristics for use in a particular device or waferstructure, and may be used for example to transmit or confine opticalradiation in use of the device or structure.

Packaging layers according to embodiments of the present invention canbe patterned in depth, for instance using masks with non-uniform opticaldensity, such as in binary or grey scale lithography, and this can beparticularly advantageous where one or more optical characteristics ofthe packaging layer is to be used. For example, one or more packaginglayers can be used in achieving alignment between components and/orstructures in a wafer or other assembly, for instance enablingappropriate waveguiding.

It is possible to construct a step change in the depth of a packaginglayer according to an embodiment of the present invention. This canprovide a very useful abutment surface for positioning a facet of anactive optical device, such as a laser, in subsequent mounting of thedevice in an assembly.

Thus, using any of a variety of fabrication methods, such as binary andgrey scale photomasks, positive and negative resist materials andappropriate etching processes, embodiments of the invention can providea wide variety of both alignment and optical characteristics which areparticularly useful in fabricating wafer-based assemblies, such asabutment surfaces and light guiding structures, including for exampletapered waveguides, index matching fluids and bonding materials, andspecialised coupling arrangements.

According to a second aspect of the present invention, there is provideda method of packaging a substrate-based assembly, which method comprisesthe step of providing a packaging layer comprising a glass materialhaving both organic and inorganic components.

A method according to the second aspect of the present invention mayoptionally comprise the step of providing one or more electricalinterconnect structures in or adjacent to the packaging layer.

Further, such a method may include the step of using bump bonding tobond a component to an electrical interconnect structure. This has theadvantage that the method may further comprise final alignment of two ormore components by manipulating at least one of the components while thebonding material is sufficiently soft to accommodate the manipulation.For instance, the method might comprise the steps of:

a) maintaining the temperature of the bump bonding material above asoftening temperature for the material and micro-manipulating thecomponent in relation to the mounting pad; and

b) lowering the temperature of the bump bonding material to below saidsoftening temperature so as to achieve bump bonding.

A method according to the second aspect of the present invention mayoptionally comprise the step of lithographic processing of the packaginglayer. Such lithographic processing might be used for example to providedepth adjustment, or to create one or more recesses or holes in thepackaging layer for use in positioning one or more components of thesubstrate-based assembly. Advantageously, embodiments of the presentinvention will support a method of fabricating a substrate-basedassembly which method comprises lithographic processing of eachfabricated layer of the substrate-based assembly. This offers veryaccurate positioning of components.

Lithographic processing to provide depth adjustment might comprise forexample the use of a lithography mask having non-uniform opticaldensity.

An advantageous method of fabricating a substrate-based assemblycomprises the steps of applying an electrical interconnect structure toa surface, applying a planarisation layer over the electricalinterconnect structure and creating one or more apertures in theplanarisation layer to give access to the electrical interconnectstructure.

Further, a method according to the second aspect of the presentinvention may optionally comprise the step of using gray scalelithography to fabricate a groove of tapered cross section in apackaging layer for mounting a fibre for optical coupling with anoptical component.

Embodiments of the present invention can provide a practicablewafer-level packaging technique for electrical and/or opticalcomponents. In particular, embodiments of the invention can provide anovel method to fabricate an optical sub assembly at wafer-level,followed by alignment, testing and chip-scale packaging of the subassembly.

Embodiments of the present invention are useful in packaging active(i.e. gain providing) optical devices. As an example, an embodiment ofthe invention can be used to manufacture chip-scale packaged tunablelaser components. However, embodiments of the invention are not limitedto use in packaging optical components but can be applied in packagingvarious electrical and optoelectrical devices. Resulting devices can bevery compact and free of electromagnetic interference (EMI) andvibrational problems. Ease of integration and testing reduces devicecost dramatically due to reducing the required equipment capital as wellas ease of testing and early failure detection. In addition, due to thecompact size and fairly dense integration level, resulting devices arereliable and can be used for example in modulation as very fastoptoelectrical devices.

BRIEF DESCRIPTION OF THE DRAWINGS

A process and assembly for wafer level packaging will now be describedas an embodiment of the present invention, by way of example only, withreference to the accompanying figures in which:

FIG. 1 shows a schematic three quarter view of a substrate for use infabricating wafer assemblies;

FIG. 2 shows the substrate of FIG. 1 with a packaging layer deposited onthe substrate;

FIG. 3 shows the substrate of FIG. 2 after lithographic processing ofthe packaging layer to produce multiple patterned structures;

FIG. 4 shows the structure of FIG. 3 after addition of a spin-on glasslayer above the patterned structures of the packaging layer;

FIG. 5 shows one of the patterned structures of FIG. 3 in more detail;

FIG. 6 shows the patterned structure of FIG. 5 with the supportingsubstrate and the spin-on glass layer;

FIG. 7 shows a vertical cross section through the patterned structureand supporting substrate of FIG. 6, generally along the optical axis ofthe sub-assembly which the structure is intended to accommodate;

FIG. 8 shows a plan view, from above, of a first (“closed”) version ofthe patterned structure of FIG. 5;

FIG. 9 shows a plan view, from above, of a second (“open”) version ofthe patterned structure of FIG. 5;

FIG. 10 shows the same vertical cross section as that shown in FIG. 7with the addition of a wire-bonded sub-assembly of components which thestructure is intended to accommodate. Both FIGS. 10A and 10B show a“closed” patterned structure, as shown in FIG. 8;

FIG. 11 shows a three quarter view from above of the wire-bondedsub-assembly shown in FIGS. 10A and 10B in place in the wafersub-assembly;

FIG. 12 shows the vertical cross section of FIG. 7 with the addition ofa bump-bonded sub-assembly of components which the structure is intendedto accommodate;

FIG. 13 indicates degrees of freedom in micromanipulation for alignmentof optical components during fabrication of a wafer sub-assembly;

FIG. 14 shows a contact pad in cross section, for use in a bump-bondedsub-assembly as shown in FIG. 12;

FIG. 15 shows a joining structure for a fibre pigtail in cross section,for use in a bump-bonded sub-assembly as shown in FIG. 12; and

FIG. 16 shows a vertical cross section of a substrate supporting anelectrical interconnect structure;

FIG. 17 shows the arrangement of FIG. 16 after application of aplanarisation layer;

FIG. 18 shows the planarisation layer of FIG. 17 after photolithographyand development or etching;

FIG. 19 shows the arrangement of FIG. 18 with electrical connectionsadded to reach the electrical interconnect structure;

FIG. 20 shows a three quarter view from above of the arrangement of FIG.19, with the addition of a tapered waveguide; and

FIG. 21 shows a vertical cross section through the arrangement of FIG.20, together with a flip chip mounted optical device coupled to thewaveguide and a further packaging layer.

It should be noted that none of the figures is intended to be drawn toscale. The figures are schematic representations only.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In general, FIGS. 1 to 11 and the description associated with themrelate to a wafer based assembly in which a packaging layer primarilyprovides mechanical protection and positioning for wire-bondedcomponents, although in the arrangement of FIG. 10B there is an opticalrequirement that the packaging layer is transparent in use. FIGS. 12 to15 relate to a bump-bonded version of the assembly of FIGS. 1 to 11.FIGS. 16 to 21 and the description associated with them relate to anassembly in which packaging layers provide mechanical protection, devicealignment in the finished assembly and optical behaviour such aswaveguiding.

Packaging Layer(s) Primarily Providing Mechanical Protection andComponent Positioning

Referring to FIG. 1, a substrate 100 suitable for use in integratingactive and/or passive components and devices might comprise silicon(CTE=3 ppm), glass (CTE=8 ppm), composite materials, ceramics includingmulti-layer ceramics such as alumina (CTE=6 ppm), and lowtemperature-co-fired ceramics (LTTC), and even conventional printedcircuit board materials such as polyimide (CTE=50 ppm) and FR-4 (CTE=16ppm). The dimensions of the substrate 100 might vary considerably, forinstance having dimensions of the order of less than one inch up toabout twelve inches. In the following example, the substrate is apre-fabricated, six-inch substrate.

Referring to FIG. 2, a packaging layer 200 of hybrid glass material isapplied to the whole substrate and pre-baked before patterning byexposure using ultra-violet (UV) light. The layer thickness may varyfrom 1 nm to 1 mm.

Example 1

In more detail, in a first method the packaging layer 200 of hybridglass material [CTE approximately 35 ppm index of refraction of 1.51 at1550 nm and dn/dT value of −3.2×10⁻⁴ (1/° C.)] is synthesized byapplying wet chemistry processing techniques (ie synthesis using liquidphase conditions). In particular, 0.1 mol of 3-glysidoxy-propyltrimethoxysilane is mixed with 0.0125 mol of tetrachlorosilane in excessof diethyl chloride. The solution is refluxed and then 0.1 mol of3-methacryloxy-propyl trimethoxysilane and 0.05 mol of tetrachlorosilaneare added to the solution. In addition, excess of ultra pure water(typically where contamination is less than 10 ppb) is added and themixture is allowed to react for several hours. Sodium hydrocarbonate isadded into the solution after which the precipitate is filtered. Thematerial is finalized by removing volatile components such as water andsolvent from the solution and 0.5 w-% of benzophenone and 0.25 w-% ofIrgacure 819 are added.

The following lists the purposes of each of the precursors mentionedabove although it should be noted that they may be multi functional:

3-Glysidoxy-Propyl Trimethoxysilane

is used to increase the CTE and produce flexibility in the matrix. Theglysidoxy part of the molecule may also be used for thermal or photopolymerisation of the material. The trimethoxy part of the precursorundergoes a hydrolysis and condensation reaction and forms a siliconoxide matrix. It thus decreases the CTE but increases the thermalstability.

Tetrachlorosilane

undergoes hydrolysis and condensation and contributes to a silicondioxide matrix in the material system. It increases thermal stabilityand decreases CTE. It may also be used as a catalyst for thering-opening polymerisation of the epoxy (e.g. glysidoxy) moieties inthe material since it acts as a Lewis acid.

3-Methacryloxy Propyl Trimethoxysilane

is used to create photosensitivity in the material. A methacryloxymoiety forms the photosensitivity through acryloxy carbon to carbondouble bond breakage and continual crosslinking polymerisation. Theorganic polymer matrix formed increases the CTE and is not as stable asan inorganic silicon oxide matrix. The trimethoxy part undergoeshydrolysis and condensation and forms a silicon oxide matrix.

Water

is used as a hydrolyzation agent for alkoxides and chlorates.

Sodium Hydrocarbonate

is used to neutralize the material which contains some free chlorineions. The carbonate reacts with free protons and thus neutralizes thesolution.

Benzophenone

is used as a thermal/photo initiator to form free radicals duringthermal/photo exposure to create methacryloxy crosslinking.

Irgacure

is used as a photoinitiator to form free radicals during photo exposureto activate methacryloxy crosslinking. Irgacure is a product of CibaSpecialty Chemicals.

The material is spun-on (or alternatively dip deposited) by applying adynamic spinning procedure. Edge bead removal is carried out by usingacetone spray. The sample is prebaked at 120° C. for 5 minutes on a hotplate in nitrogen.

Example 2

In a second method, the packaging layer 200 of hybrid glass material[CTE approximately 15 ppm, index of refraction of 1.50 at 1550 nm anddn/dT value of −2.4×10⁻⁴ (1° C.)] is synthesized as follows. 0.1 mol ofphenyltrichlorosilane, 0.1 of 3-methacryloxypropyl trichlorosilane and0.016 mol of trimethylolpropane trimethacrylate are mixed and hydrolysedin the presence of dichloromethane and ultra pure water. The mixture isreacted for a time (from 1 hour to 48 hours) sufficient to allow all thesilanes to hydrolyze. The organic solvent phase is separated from thesolution. The material is finalized by removing volatile components suchas water and solvent from the solution and 0.5 w-% of benzophenone and0.25 w-% of Irgacure 819 and Irgacure 184 are added.

In this second method:

Phenyltrichlorosilane

is used to increases the CTE from silicon dioxide values. The phenylmoiety is highly stable and also provides physical flexibility andelasticity in the resulting material. It hydrolyses and condenses toform a silicon oxide matrix.

3-methacryloxypropyltrichlorosilane

is used to create photosensitivity in the material. A methacryloxymoiety forms the photosensitivity through acryloxy carbon to carbondouble bond breakage and continual crosslinking polymerisation. Theorganic polymer matrix formed increases the CTE and is not as stable asan inorganic silicon oxide matrix. The trichloro part undergoeshydrolysis and condensation and forms a silicon oxide matrix.

Trimethylolpropane Trimethacrylate

is used to increase the photosensitivity of the material duringexposure. The three methacryloxy moieties in the molecule participate inthe free radical polymerisation and result in a highly crosslinkedorganic matrix. The organic polymer matrix formed increases the CTE andis not as stable as an inorganic silicon oxide matrix.

Benzophenone

is used as a thermal/photo initiator to form free radicals duringthermal/optical exposure to create methacryloxy crosslinking.

Irgacure

is used as a photoinitiator to form free radicals during photo exposureto activate methacryloxy crosslinking. Irgacure is a product of CibaSpecialty Chemicals.

The material is spun-on (or alternatively dip deposited) by applying adynamic spinning procedure and edge bead removal is carried out by usingacetone spray. The sample is prebaked at 120° C. for 10 minutes on a hotplate in nitrogen.

Example 3

In a third method, the packaging layer 200 of glass material [CTEapproximately 22 ppm, index of refraction of 1.48 at 1550 nm and dn/dTvalue of −1.8×10⁻⁴ (1/° C.)] is synthesized as follows. 0.1 mol ofphenyltrichlorosilane and 0.1 mol of trichlorovinylsilane are mixed andhydrolyzed in the presence of dichloromethane and ultra pure water. Themixture is reacted for a time (from 1 hour to 48 hours) sufficient toallow all the silanes to hydrolyze. The organic solvent phase isseparated from the solution. The material is finalized by removingvolatile components such as water and solvent from the solution and 0.5w-% of benzophenone and 0.25 w-% of Irgacure 819 and Irgacure 184 areadded.

In this third method:

Phenyltrichlorosilane

is used to increases the CTE from silicon dioxide values. The phenylmoiety is highly stable and also provides flexibility for the resultingmaterial. It hydrolyses and condenses to form a silicon oxide matrix.

Trichlorovinylsilane

is used to create photosensitivity in the material. A vinyl moiety formsthe photosensitivity through the carbon to carbon double bond breakageand continual crosslinking polymerisation.

The organic polymer matrix formed increases the CTE. The organic matrixis not as stable as an inorganic silicon oxide matrix. The trichloropart undergoes hydrolysis and condensation and forms a silicon oxidematrix.

Benzophenone

is used as a thermal/photo initiator to form free radicals duringthermal/photo exposure to create methacryloxy crosslinking.

Irgacure

is used as a photoinitiator to form free radicals during photo exposureto activate methacryloxy crosslinking. Irgacure is a product of CibaSpecialty Chemicals.

The material is spun-on (or alternatively dip deposited) by applying adynamic spinning procedure and edge bead removal is carried out by usingacetone spray. The sample is prebaked this time at 150° C. for 5 minuteson a hot plate in nitrogen.

As mentioned earlier in this specification, if high processingtemperatures are required the organic component content of the packaginglayer 200 should be kept to a minimum. Low processing temperatures aremade possible by using thermal or photo initiation of the organicmatrix. In Examples 1 to 3 described above, three different hybrid glassmaterial compositions and synthesis procedures are described that can beprocessed at various temperatures. Material synthesized according toExample 1 requires a processing temperature of not more than 200° C.Material synthesized according to Example 2 requires a processingtemperature of not more than 150° C. Material synthesized according toExample 3 allows baking temperatures up to 450° C. or so. This can beparticularly advantageous for example where a soldering operation is tobe carried out since the materials may then be subjected to relativelyhigh temperatures.

Referring to FIG. 3, the packaging layer 200 is patternedlithographically by exposing it to UV light and developing it with achemical developer. The layer 200 may also be treated at elevatedtemperature to increase the density of the glass material. The result isa plurality of patterned structures 300 which contain features for usein integration that are shown in more detail in FIGS. 5 to 9.

In more detail, the packaging layer 200 is UV (I-line) exposed through adark-field contact mask which contains both binary and gray-scalefeatures. The packaging layer 200 is then spray developed with a mixedethanol methyl isopropyl ketone developer. The material acts as anegative-tone photoresist. This part of the process is finalized bybaking at 200° C. for 2 hours in nitrogen.

Gray scale lithography, for producing gray-scale features, is a knowntechnique described for example in the following: “Fabrication ofMicro-Optical Structures by Applying Negative-Tone Hybrid GlassMaterials and Greyscale Lithography”, by A. H. O. Käkkäinen, J. T.Rantala, M. R. Descour, published in Electronics Letters, Vol. 38, No.1, pp 23-24 (2002).

The process just described can be used to produce a thick assemblystructure (for instance in the range from 1 micron to 1 mm) bylithographic means. This structure might for instance have recesses orholes in which contact pads and optical and/or electrical components canbe positioned in relation to one another and these are further describedwith reference to FIGS. 5 to 9.

Referring to FIG. 4, once the components and interconnect material arein place, in or on the packaging layer 200, an optional furtherpackaging layer 400 can be deposited for further protection andstrength, using a spin-on glass procedure followed by processing at lowtemperature at wafer level. This further packaging layer 400 can againbe of hybrid glass but conventional polymers such as polyimides orepoxides can also be used. Typically, the further packaging layer 400 ismade of a material which is an optically different mix from that of theearlier packaging layer 200 to produce a lower refractive index in thefurther packaging layer 400. This lower refractive index allows thefurther packaging layer 400 to provide optical confinement for instanceof an evanescent field associated with optical radiation travelling inthe assembly in use. The thickness of the further packaging layer 400will usually be at least 1 micron but in any event preferably thickenough that optical and/or electrical components already mounted areburied by it.

(The patterned structures are shown in dotted outline in FIG. 4.However, this should not be taken to mean that the further packaginglayer 400 is opaque in visible light. It may or may not be opaque invisible light.)

Referring to FIG. 5, each one of the patterned structures 300 of FIG. 3is a patterned hybrid glass structure that contains a set of recesses orholes, each one of which provides an integration location for acomponent or other element of an assembly. For example, each recess orhole as shown in FIG. 5 might be dimensioned and positioned to providethe following:

-   -   photodetector recess 500    -   gain element recess 505    -   tunable reflection element recess 510    -   optical isolator recess 515    -   thermo element or temperature sensor recess 520    -   fibre groove 525    -   integration or contact pads 530

An assembly with its components in place (see FIGS. 10A and 10B) mightfor example be used as a tunable optical source of the type described inU.S. patent application Ser. No. 10/046,914 assigned to Optitune plc. Insuch an optical source, a tunable element 1010, such as anelectro-optically controlled zone plate device or a Fabry Perot elementwhich can be moved using a micro-electromechanical system (MEMS), isused to return optical radiation of a selected wavelength to a tunablelaser 1015. A photodiode 1020 can be used to monitor the laserperformance and the output of the assembly can be picked up by a fibre1000 located in the fibre groove 525.

Other ways of tuning an optical source that might be used in anembodiment of the present invention include a tunable thin film filter,a micro-prism, a prism-grating combination or a prism-grating-prismdevice.

As well as the directly functional parts of an assembly as describedabove, it is also possible to embed control electronics such as thermalcontrollers, based on for example a thermo element or temperature sensor520 which is also locatable at a hole or recess in the packaging layer200 of the glass material. This becomes available because of the verylow processing temperatures, for instance less than 200° C., at whichthe hybrid glass material can be deposited. Hence the packaging layer200 can be fabricated on top of metallic structures which typically havea melting point around 350° C. or more. Conventional glass manufacturingtechniques cannot be used in such circumstances due to the fact thatthey typically require temperatures of more than 800° C. to consolidate.

Another significant advantage of the hybrid glass materials as apackaging material is that conventional glass materials cannot bedirectly lithographically patterned after deposition. The use of ahybrid glass material which has lithographic patterning capabilityremoves a significant number of processing steps in manufacture.

Although described primarily as being located in the packaging layer 200as described above, control electronics can alternatively be fabricatedeither into the substrate 100 or on top of the substrate 100. If theyare located in the hybrid glass packaging material, that might be by useof holes or recesses as described above and/or by the use of a secondhybrid glass material deposition. This is shown as a further layer 400in FIG. 6.

The substrate 100 may also include optically active device such as taps(photodiodes), laser drivers and wavelength reference devices. Theymight be integrated inside the substrate or in layers deposited abovethe substrate.

Interconnect material is present in the packaging layer 200 as theintegration or contact pads 530. Such interconnect material is of knowntype and configuration and might comprise for example copper (CTE 17ppm) or aluminium (CTE 23 ppm). To make electrical connection, theintegration or contact pads 530 can be provided on thin pads at thesurface of the substrate 100 or integrated into a layer provided for thepurpose. To make electrical connection between the contact pads 530 andcomponents, wire bonds 1100 (shown in FIG. 11) can be used.

In an alternative arrangement, electrically interconnecting mountingpads 1200 can be positioned in the component recesses 500, 505, 510,515, 520 and the known technique of “solder bump bonding” can be used.This is shown in FIG. 12.

In general, fabrication and connection of the mounting or contact pads530, 1030 is further discussed below with reference to FIGS. 13 to 15.

After creation of the patterned hybrid glass packaging layer 200 withits mounting or contact pads 530, 1030, the layer(s) can be “back”polished using a chemical and/or mechanical polishing technique.Electrical and/or optical components can then be mounted to create awafer assembly. Such components might include for example a gainelement, photodetector, reflecting element, lenses (ball or graded indextype), optical isolator, and a thermo-element.

Referring to FIG. 7, a cross section long the line “A-A” in FIG. 6,viewed in the direction indicated by the arrows, shows lithographicallydefined holes in the packaging layer 200 for the following:

-   -   photodetector hole 500    -   gain element hole 505    -   tunable reflection element hole 510    -   optical isolator hole 515

The fibre V-groove 525 is also shown. Such a groove can be fabricatedusing known gray scale lithography techniques. Although shown in FIG. 7as a hole, the depth of the groove will of course be tailored to thedimensions of the fibre end to be located therein and might be forinstance of the order of 50 μm deep.

Referring to FIGS. 8 and 9, the patterned packaging layer may beprovided with a “closed” integration structure, as shown in FIG. 8, oran “open” integration structure, as shown in FIG. 9. That is, the holesor recesses for the various components might be designed as physicallydiscrete and separate features in the packaging layer 200 or they may beinterconnected.

A primary difference between the closed and open structures in practiceis the presence or absence of the packaging layer 200 in the opticaltransmission path through the components. In the closed structure, ifthe packaging layer 200 is deep enough, it will lie in at least part ofthe optical transmission path between the components. This can beunderstood particularly with reference to FIGS. 10A and 10B which showcomponents 1020, 1015, 1010, 1005 located in appropriate holes in thepackaging layer 200. In FIG. 10A, the components are all aligned along acommon optical axis 1025 but the optical axis lies above the packaginglayer 200 and the optical transmission path of the assembly does notpass through any part of the packaging layer 200. In a closed structureas shown in FIG. 8, if the packaging layer 200 were deeper relative tothe components, the common optical axis 1025 would pass through it. Thishas implications for the choice of packaging layer material sinceoptical properties such as transparency and refractive index couldbecome important. Such a configuration is shown in FIG. 10B.

Referring to FIG. 10A, this shows a cross section similar to that ofFIG. 7 but with components located to their appropriate holes in thepackaging layer 200. In particular, the following components can be seenlocated:

-   -   photodetector 1020    -   gain element 1015    -   tunable reflection element 1010    -   optical isolator 1005    -   single mode optical fibre 1000

These components 1000, 1005, 1010, 1015, 1020 are mounted on integrationpads 1030 and aligned along a common optical axis 1025 which lies abovethe packaging layer 200. The packaging layer 200 might thus be depositedas a closed structure regardless of its optical characteristics, asshown in FIG. 8, although it may have for example guiding or absorptioncharacteristics which need to be taken into account in use. The closedstructure has an advantage in providing improved protection to thesubstrate in comparison with an open structure as shown in FIG. 9. Thepackaging layer 200 can provide at least coarse alignment of the opticalcomponents as well as mechanical protection.

The integration pads 1030 are constructed from any suitable materialwhich will adhere to the components and other materials it will be incontact with. The pads 1030 may or may not be electrically conductive,as required.

In FIG. 10A, both a gain element 1015 and a separate tunable reflectionelement 1010 are shown. These can be used together to provide a tunableoptical source. The gain element 1015 might be an external cavity,single mode semiconductor laser whose output can be tuned by tuning thefeedback from an external cavity created by the tunable reflectionelement 1010. For example, the tunable reflection element 1010 mightcomprise a diffraction device arranged in a Littrow configuration, thediffraction device comprising a thermo-optic material which can bethermally controlled for tuning.

In practice, the tunable reflection element 1010 might be replaced orsupplemented by the use of an external optical modulator such as anamplitude, frequency and/or intensity modulator. More specifically, forexample, an external modulator might be a Mach-Zehnder modulator, asemiconductor multiple quantum well modulator or an electrorefractionmodulator.

Referring to FIG. 10B, the gain element 1015 and separate tunablereflection element 1010 might be replaced by a monolithic tunable source1035 such as that disclosed in U.S. Pat. No. 6,041,071 in the nameCoretek Inc, or that disclosed in U.S. Pat. No. 6,275,317 in the nameAgere Systems Optoelectronics Guardian Corp. Another example of anintegrated form of tunable source 1035 is disclosed in copending U.S.patent application Ser. No. 10/046,914 assigned to Optitune PLC.

Referring to FIG. 11, metallic interconnect pads 530 are also positionedin holes in the packaging layer 200. These contact pads 530 may be madeof metals known to be suitable for bonding using wire bonding.Electrical connection to the components 1000, 1005, 1010, 1015 can thenbe provided as necessary using wire bonds 1100.

The components 1000, 1005, 1010, 1015 can largely be mounted and thewire bonds 1100 put in place before a wafer assembly is diced intoindividual assemblies. It is only generally possible to mount the fibre1000 to its groove after dicing however.

Referring to FIG. 12, instead of using wire bonds 1100 the components1000, 1005, 1010, 1015 can be mounted using the known technique of“solder bump bonding”. In this technique, electrical interconnection isprovided by mounting pads 1200 in the holes in the packaging layer 200under the components. The components 1000, 1005, 1010, 1015 are providedwith solder bumps 1205 on their bonding surface and these solder bumps1205 are brought into contact with the interconnecting mounting pads1200 and heated to form a bond between the component and the mountingpad 1200. After the heat treatment, the solder bumps are spread intointimate contact with the components 1000, 1005, 1010, 1015 and themounting pads 1200 and can provide particularly good electricalperformance.

There are variations in solder bump bonding which can also be used in anembodiment of the present invention. For example the solder bumps 1205can be provided on interconnect material in the mounting pads 1200instead of or as well as on a bonding surface of the components. In“double bump bonding”, two layers of bumps are provided, one on top ofthe other.

The electrical contact pads 530 and interconnecting mounting pads 1200can be fabricated by post-processing of the substrate 100 and packaginglayer 200 and this is further discussed below with reference to FIGS. 13to 15.

The use of bump bonding provides thermo-mechanical as well asenvironmental protection for interconnects in the sub assembly layer.Optical components attached using the bumps are protected from shearstresses. Furthermore, this technique eliminates use of an underfillprocess which might otherwise be necessary for die or componentattachment, which makes the processing procedures less complicated.

In practice, the use of the solder bumps 1205 and mounting pads 1200also makes it possible to tune the positioning of optical componentseven after they are bonded to electrical interconnect material. Thebonding material can be slightly heated so that it softens toaccommodate movement of the components, for instance on a hot chuck orby using a laser or the like. Movements of a few hundred nanometers arepossible in lateral positioning and movements of a few tens ofnanometers are easily achievable in vertical position tuning withoutcausing high stresses in the component structures.

Referring to FIG. 13, an optical component such as a gain element 1015can be mounted on a pair of contact pads 1200 by means of bump bonding.It can then be manipulated about vertical and/or horizontal planes bythe use of for example a piezo or stepper motor controlled,nano-positioning stage “push” device while the solder material betweenthe component 1015 and the contact pad 1200 is softened by the use ofheat.

Referring to FIG. 14, in more detail, each contact pad 1200 might be putdown onto a thin film pad 1400 of a material such as solder which hasbeen provided on or in the substrate 100. The contact pad 1200 can forexample be evaporated or grown electrolytically through a window in aphotoresist mask. Its dimensions might be for example 5 μm×5 μm in crosssection with a depth in the range 0.5 to 100 μm, depending on therequired position of the component to be mounted on it. The material ofthe contact pad might be for example a metal such as aluminium, gold,tin, molybdenum, nickel, platinum or copper but it may contain more thanone layer and more than one material. The contact pad 1200 of FIG. 14shows three layers.

Referring to FIG. 15, another type of structure which might require tobe mounted is a joining structure 1505 for a fibre pigtail (not shown).This might comprise the joining structure, mounted on a pad 1510supported by the substrate 100. The joining structure at the same timesupports a ball lens 1500. The fibre pigtail can then be mounted to abutthe joining structure 1505 such that its optical axis is aligned withthe lens 1500.

Optically Active Packaging Layers

Referring to FIGS. 16 to 21, a packaging layer in an embodiment of theinvention can do considerably more than provide mechanical protectionand alignment. These figures illustrate steps in fabricating anarrangement in which the packaging layers also provide opticalproperties in a finished assembly.

The materials used in providing optical properties can be of the samegeneral type as described above, for example with reference to FIGS. 2,3 and 4, and detailed examples are not therefore described below.However, clearly it is necessary to use materials having appropriateoptical characteristics and these can be achieved as already discussedabove.

Referring to FIG. 16, a first step in fabrication of an assembly is todeposit an electrical interconnect structure 1600 on a substrate 100.This might have a thickness of for instance 1 μm.

Referring to FIG. 17, a planarisation layer 1700 is deposited, coveringthe interconnect structure 1600. Planarisation can be achieved at 90%,95% or even 99%, providing a uniform surface for subsequent lithographysteps, using a layer which is 5 μm thick.

Referring to FIG. 18, the planarisation layer 1700 can be patternedlithographically. Using a lithographic mask having areas of differentoptical densities, firstly a step 1805 is created in the depth of theplanarisation layer 1700 and secondly holes 1800 are created in thethinner portion of the planarisation layer 1700, exposing theinterconnect structure 1600. The patterning can be done either by usingjust a single lithography step, grayscale photomask and negative orpositive tone material or by using a separate, subsequent photoresistlayer(s) and carrying out the patterning by using etching of the layer1700 to a desired shape. The step 1805 effectively now divides theoverall assembly into two portions: a first, thinner portion having theinterconnect structure 1600 and a second, thicker portion without.

FIG. 18 represents just one example of how the patterning and exposingthe interconnect structures 1600 can be done. The interconnectstructures 1600 can alternatively be exposed in their full width if itis preferably for the following fabrication steps such as bump bonding.

Referring to FIG. 19, the holes 1800 are filled with an electricallyconductive material 1900 to enable electrical connection of a devicemounted on the first portion of the assembly.

Referring to FIG. 20, it can be seen that the step 1805 does not in factextend right across the assembly but provides an end to a rectangulardepression in the planarisation layer 1700, over the interconnectstructure 1600. A waveguide structure 2000, 2005 is now constructed.

Although other arrangements might be suitable in other assemblies, inthe arrangement shown in FIGS. 6 to 21, a semiconductor laser 2100, 2105is to be mounted in the rectangular depression and the waveguidestructure 2000, 2005 provides a specialised coupling arrangement todeliver optical radiation from the laser output facet to anothercomponent In particular, the purpose of the waveguide structure 2000,2005 is to change the dimensions of the laser beam's optical mode crosssection, in adiabatic fashion, prior to delivery to the other component.The waveguide structure 2000, 2005 therefore has a mode taper section2000 and a cylindrical waveguiding section 2005 having a square orrectangular cross section. (Tapered structures of this sort are knownfor mode-size matching, for instance in both laser sources and passivewaveguide devices, and the specific design characteristics of thewaveguide structure 2000, 2005 are not therefore further discussedherein.)

The waveguide structure 2000, 2005 is fabricated using techniques whichcan control both depth and the dimensions parallel to the supportingsurface of the substrate 100. Firstly the material of the waveguidestructure 2000, 2005 is spun onto the assembly, then etched using alithographic mask with both binary and gray-scale features. Thegray-scale features will produce the gradual change in depth along themode taper section 2000 while the binary features will produce the shapeof the waveguide structure 2000, 2005 in plan view.

In an alternative approach to producing the planarisation layer 1700 andthe waveguide structure 2000, 2005, the materials for both can be spunin turn onto the substrate 100 and then etched using a mask having acombination of binary and gray-scale features to achieve the desiredconstruction.

Referring to FIG. 21, this shows a vertical cross section along theoptical axis of a completed assembly. Here, a semiconductor laser havingoptical layers 2100 supported by a substrate 2105 has been flip-chipmounted into the rectangular depression in the planarisation layer 1700,over the interconnect structure 1600. A facet of the laser 2100, 2105abuts the step 1805 in the planarisation layer 1700 and, in use, anoutput beam 2110 is delivered to the waveguide structure 2000, 2005. Thelaser 2100, 2105 can be driven electrically through the interconnectstructure 1600. Lastly, a further packaging layer 2115 has been appliedacross the assembly.

Flip chip mounting is a known technique in which a device or assemblywhich has been fabricated on a substrate is then inverted and mountedonto another substrate for connection to other devices or components.The result is that the device or assembly is sandwiched between its ownsubstrate and the substrate carrying the other components. A techniqueof this general sort is disclosed in U.S. Pat. No. 5,478,778, and inco-pending British patent application GB 225,522.2, in the name Optituneplc.

Where the device or assembly is optical, this can have advantages inease of optical alignment. The basic principle is that the opticalconfinement regions for the first and second components can be veryaccurately matched during fabrication to achieve accurate opticalcoupling of the two when inverted in the substrate based assembly. Thethickness of the respective substrates for the first and secondcomponents, which is far less controllable, is not relevant in thealignment which is determined instead by the thickness of confininglayers constructed during fabrication. One of the components mayalternatively be fabricated on the substrate based assembly using planarfabrication techiques prior to assembly of the other. The distances fromthe bonding surfaces to the optical confinement regions for the firstand second components are matched and flip chip mounting producespassive alignment in the direction normal to the bonding surfaces.Alignment in the other two directions, in a plane parallel to thebonding surfaces, can be achieved using known alignment techniques.

Specific optical characteristics of the materials selected for anassembly as described above and shown in FIG. 21 are not described here.Clearly, the refractive index of layers such as the waveguide structure2000, 2005 and the packaging layers 1700, 2115 above and below it wouldhave to be selected so that waveguiding is achieved. However, waveguidesand factors in their design are known and widely documented.

It will be understood that many variations and changes can be made inassemblies made according to an embodiment of the invention. A minorchange might be to use different techniques for lithography and etching.However, in general, techniques including those described above can beused in producing a wide variety of wafer-based assemblies. For example,a substrate based assembly can be used to fabricate and assemble:

-   -   various light source elements such as laser diodes and fibres        which need to be coupled to elements or components external to        the substrate based assembly    -   light emitting diodes (organic and inorganic)    -   free space optics components such as lenses, beam splitters,        prisms, gratings, and the like    -   detectors such as charge coupled devices and CMOS devices

It might be noted that a substrate based assembly might be much morecomplex than as shown in the Figures. For example, in FIG. 21, the laser2100, 2105 is shown with its rear facet adjacent the edge of thesubstrate 100. This is not essential. There may be components mounted onthe substrate 100 behind the laser for monitoring or other purposes. Thepackaging layers 1700, 2005 and 2115 might be used to providewaveguiding behind the laser as well as in front.

Any of these components can potentially be fabricated first on aseparate substrate and then assembled into the substrate based assemblyof the invention.

A further function available in using the hybrid glass materials aspackaging layers is as index matching material, including fluids, foruse in assembling and aligning the confinement regions of opticalcomponents. The refractive index of the hybrid glass materials usedshould preferably be lower than the index of the optical confiningmaterial (e.g. waveguide core) and higher than air. This can beoptimized in each case. The index matching material can also be madephotocurable and serve at the same time as an additional bonding layerof the two optical components. The two optical components can be firstaligned together and then the UV-curable index matching fluid is appliedand cured using Uv-light (and/or heat).

What is claimed is:
 1. A substrate-based assembly for carrying opticalcomponents, the assembly comprising: a substrate; at least one activeoptical component and at least one passive optical component mounted onthe substrate; a packaging layer formed on the substrate for protectingsaid optical components, the optical components and the packaging layerbeing carried by the substrate; wherein the packaging layer comprises atleast two packaging layers, each of said packaging layers being at leastone of (a) a planarization layer or (b) a passivation layer, at leastone of the packaging layers comprising a hybrid glass material havingboth organic and inorganic components and being provided with aplurality of recesses, wherein each of the recesses has a wall surfacetherewithin supporting at least one of the optical components, eachrecess having a perimeter which is substantially closed, into which atleast one of the components is mounted; and wherein the refractive indexof a first of the at least two packaging layers is different from therefractive index of either a second of the at least two packaging layersor at least one of the optical components.
 2. The substrate-basedassembly according to claim 1, wherein the hybrid glass materialincludes an organic component which polymerizes by cross-linking.
 3. Thesubstrate-based assembly according to claim 1, wherein the hybrid glassmaterial includes an organic component which polymerizes under thermalor photo treatment.
 4. The substrate-based assembly according to claim1, wherein the hybrid glass material includes at least one of an epoxycomponent, aluminium oxide and silicon oxide.
 5. The substrate-basedassembly according to claim 1, wherein the hybrid glass materialcomprises an inorganic matrix provided at least in part by a metalalkoxide or salt, the metal alkoxide or salt each being hydrolyzed inprovision of the inorganic matrix.
 6. The substrate-based assemblyaccording to claim 5 wherein the metal alkoxide or salt is based ongroups 3A, 3B, 4B and/or 5B of the Periodic Table.
 7. Thesubstrate-based assembly according to claim 1, wherein the hybrid glassmaterial includes at least one hydrocarbon compound from the groupcomprising acrylates, epoxides, alkyls, alkenes, and aromatic groups. 8.The substrate-based assembly according to claim 1 wherein thecoefficient of thermal expansion of the at least one packaging layerapproaches that of the substrate material.
 9. The substrate-basedassembly according to claim 1, wherein the at least one recess containselectrical interconnect material for providing electrical connection toat least one component packaged by the packaging layer.
 10. Thesubstrate-based assembly according to claim 9 wherein the coefficient ofthermal expansion of the at least one packaging layer approaches that ofthe electrical interconnect material.
 11. The substrate-based assemblyaccording to claim 9 wherein the coefficient of thermal expansion of theat least one packaging layer differs from the coefficient of thermalexpansion of the electrical interconnect material and/or the substratematerial by not more than 15 parts per million.
 12. The substrate-basedassembly according to claim 9 which further comprises at least onecontact pad for a wire bond to the at least one component, theelectrical interconnect material being present in said contact pad orwire bond.
 13. The substrate-based assembly according to claim 9 whichfurther comprises at least one mounting pad for mounting the at leastone component, the electrical interconnect material being present insaid mounting pad.
 14. The substrate-based assembly according to claim1, wherein said at least one optical component comprises a bump bondedoptical component.
 15. The substrate-based assembly according to claim 1wherein the material of the at least one packaging layer islithographically patterned.
 16. The substrate-based assembly accordingto claim 15 wherein the material of the at least one packaging layercomprises at least one organic material which photopolymerizes, the atleast one organic material being selected from the group comprisingacrylates, epoxides, alkyls, alkenes, and aromatic groups.
 17. Thesubstrate-based assembly according to claim 1 wherein the packagingmaterial has a processing temperature of not more than 450° C.
 18. Thesubstrate-based assembly according to claim 1 wherein the packagingmaterial has a processing temperature of not more than 200° C.
 19. Thesubstrate-based assembly according to claim 1 wherein the packagingmaterial has a processing temperature of not more than 150° C.
 20. Thesubstrate-based assembly according to claim 17 wherein the packagingmaterial is fabricated from a material comprising a polymerisationinitiator.
 21. The substrate-based assembly according to claim 1 havinga substrate comprising at least one material from the group comprisingsilicon, glass, composite materials, ceramics and printed circuit board.22. The substrate-based assembly according to claim 1 wherein the atleast one packaging layer is a planarization layer.
 23. Thesubstrate-based assembly according to claim 22 wherein the planarizationlayer provides waveguiding.
 24. The substrate-based assembly accordingto claim 1 wherein at least one of the recesses comprises an aperture togive access to an electrical interconnect structure.
 25. Thesubstrate-based assembly according to claim 22 wherein one or morecomponents is/are mounted at least partially on the planarization layer.26. The substrate-based assembly according to claim 1 wherein therefractive index of a first of the at least two packaging layers isdifferent from the refractive index of a second of the at least twopackaging layers.
 27. The substrate-based assembly according to claim 1wherein the at least one packaging layer transmits optical radiation.28. The substrate-based assembly according to claim 1 wherein thepackaging layer provides an alignment feature for use in aligning anoptical component in the assembly.
 29. The substrate-based assemblyaccording to claim 1 wherein the packaging layer, provides refractiveindex matching in use of the assembly.
 30. The substrate-based assemblyaccording to claim 1 wherein the packaging layer, provides bondingbetween optical components in the assembly.
 31. The substrate-basedassembly according to claim 1 wherein the active optical componentcomprises a laser or a tunable optical source.
 32. The substrate-basedassembly according to claim 1 wherein at least one of the recessescomprises an aperture to give access to an electrical interconnectstructure and wherein the active optical component is bump-bonded orflip-chip mounted in the assembly to connect to the electricalinterconnect structure.
 33. The substrate-based assembly according toclaim 32, wherein the active optical component comprises a laser or atunable optical source, the assembly further comprising an opticalmodulator, external to the laser or tunable optical source.
 34. Thesubstrate-based assembly according to claim 1 wherein thesubstrate-based assembly comprises a thick substrate-based assembly. 35.The substrate-based assembly according to claim 1 wherein thesubstrate-based assembly has a thickness in the range from 1 micron to 1millimetre.
 36. Opto-electronic equipment comprising the substrate-basedassembly according to claim
 1. 37. The substrate-based assembly as inclaim 1, wherein the packaging layer provided with said one or morerecesses is otherwise substantially continuous.